Progress has been made in recent years in developing bicycles which comprise a bicycle body drivable by human power and an electric motor mounted on the body for supplementing the drive force afforded by human power. (The electrically drivable bicycle, i.e., the bicycle equipped with an electric motor, will hereinafter be referred to as an "electric bicycle.")
FIG. 12 schematically shows the construction of an electric bicycle. As illustrated, an electric motor 3 is provided in the vicinity of the hub axle 19 of a rear wheel 12, and has an output shaft (not shown) coupled to the hub axle 19, for example, via a series of three speed reduction mechanisms (not shown). Each of the reduction mechanisms comprises, for example, a drive pulley, driven pulley and belt.
The human power torque generated by operating the pedals 17 on the body 1 of the bicycle is transmitted to the hub axle 19 of the rear wheel 12 by way of a human power transmission mechanism 2 which comprises a drive sprocket 21, chain 22 and driven sprocket 23.
The human power torque produced by operating the pedals 17 of the bicycle body 1 is detected by a torque sensor 40, and the resulting detection signal is fed to a controller 4, which in turn prepares a torque command in accordance with the input signal and gives the command to the motor 3. Consequently, the rotation of the motor 3 is subjected to speed reduction successively by the series of three speed reduction mechanisms and transmitted to the hub axle 19 of the rear wheel 12.
In this way, the rotation of the motor 3 is delivered to the rear wheel 12 on speed reduction to supplement the human drive force.
With electric bicycles, the speed of rotation of the electric motor 3 must be reduced generally to not greater than 1/20. However, since the bicycle needs to be compacted in its entirety, it is impossible to use driven pulleys of increased diameter for the speed reduction mechanism, such that the speed reducing ratio available by one reduction mechanism is as small as 3:1. In the prior art, therefore, the speed of rotation of the motor 3 is reduced successively by at least three speed reduction mechanisms to attain a speed reducing ratio of at least 20:1. This results in the problem of making the entire bicycle complex in construction and difficult to maintain.
JP-A-89483/1983 discloses an electric bicycle comprising an electric motor and a speed reduction mechanism which are provided inwardly of the outer periphery of chain of the human power transmission mechanism and which are surrounded by a complex structure. The bicycle therefore has the problem of difficult maintenance.
JP-A-262273/1993 and JP-A-278670/1993 disclose electric bicycles comprising an electric motor and a speed reduction mechanism which are arranged in the vicinity of the hub axle of the rear wheel. Accordingly, the driven sprocket and chain of the human power transmission mechanism must be removed when the motor and the reduction mechanism are to be inspected or repaired. Thus the disclosed bicycles have the problem of necessitating a cumbersome procedure for inspection or repair.
JP-A-291175/1995 and JP-A-329873/1995 disclose electric bicycles wherein a driven pulley providing a speed reduction mechanism has a large diameter, which nevertheless is only about 1/2 of the diameter of the rear wheel. The mechanism therefore has the problem of failing to give a sufficiently great speed reducing ratio. The mechanism further has a problem with respect to strength because the driven pulley is attached to spokes of the rear wheel.